Unlike incandescent lamps and electric-discharge lamps, LEDs are readily switched on with a direct-current power supply and have a long lifespan. Recently, blue and white LEDs have been put to practical use. Efficiency of LEDs also has reached a level that is better than efficiency of not only incandescent lamps but also fluorescent lamps. LEDs are increasingly widely used not only as display light sources of indicators or the like but also as ordinary illumination light sources or the like. Constant brightness is sufficient for ordinary illumination, whereas dimming for changing the brightness as desired is desirable for some types of interior illumination, stage illumination, and effects illumination at facilities such as planetariums. Changing the brightness of interior illumination as desired allows people to enjoy a change in atmosphere. At bedtime, dim illumination can substitute a function provided by traditional night light bulbs or the like. Stage illumination not only can express a brightness change corresponding to a scene but also can create various scenes by changing the brightness of light sources of individual colors such as red, green, and blue to control hue and saturation. At planetariums, the color and the brightness of illumination are changed as desired not only to use the illumination as houselights when an audience enters or exits but also to express daytime and nighttime scenes. A subtle change in brightness from daytime to dusk and to nighttime is expressed by changing the brightness of illumination.
To dim an LED, a method of changing drive current, a method based on PWM (pulse width modulation) control, and other methods are used. In general, the method based on PWM control is often used because of simple circuitry. Since the amount of LED light responds to current in 1 ms or less, which is extremely fast, LEDs have a property in which the brightness and the pulse width are directly proportional to each other. This property enables accurate brightness control.
However, in the case where LEDs are used in the aforementioned illumination applications, an issue that is not caused by the hitherto used light sources such as incandescent lamps may occur. The issue is that the brightness change is perceived as a stepped change at low illuminance. In PWM control, a command regarding a pulse width can be numerically given, and the pulse width can be accurately specified using a digital circuit. For example, in the case of a PWM signal of 12-bit resolution, the minimum brightness is 1/4,096. Although this brightness seems extremely low, even this minimum brightness is sufficiently bright for humans because the dynamic range of the human eye for brightness is extremely wide. For example, suppose that the maximum brightness is 500 lx. In this case, the minimum brightness of 1/4,096 is equal to 0.12 lx, which is sufficiently bright to be sensed by eyes. When the brightness is increased by one step, the brightness is equal to 0.24 lx, that is, is steeply doubled. Consequently, the stepped brightness change is visually noticeable.
To address this issue, a solution for increasing the resolution of the PWM signal has been conceived. For example, in the case of 16-bit resolution, the minimum brightness is equal to 0.0076 lx, and a significant improvement is expected. However, increasing the resolution in this manner may cause another issue. Specifically, in the case where PWM control is performed at a brightness at which no flicker is noticeable, for example, at approximately 200 Hz, one period is 5 ms. When control is performed at 16-bit resolution, the minimum pulse width is equal to 5 ms/65,536=76.3 ns. Although this is not a speed that is difficult to handle as a speed of a signal of digital circuits, noise is likely to occur if current for causing an LED to emit light is repeatedly supplied and cut at this speed. Consequently, neighboring electronic devices may be affected by noise or noise suppression may become difficult. In addition, if the speed at a rise or fall of the current waveform changes due to a factor such as external noise, the brightness at the lowest illuminance may vary or a flicker may occur, making the brightness change unpleasant to see.
These issues are overlooked in ordinary illumination applications as requirements therefor are met even if such issues occur. However, particularly in applications in which brightness control at low illuminance is important, for example, illumination at planetariums or the like, a subtle nighttime brightness needs to be reproduced by an illumination device. Thus, the above-described issues hinder the stage effects.
Many proposals such as LED driver circuits including a LED dimmer circuit unit for changing the brightness of an LED by using PWM control have been made. Such driver circuits are used in illumination devices, on-vehicle illumination devices, or the like. However, none of many hitherto proposed LED driver circuits are capable of exerting effects assumed by the inventor of this application. Specific issues will be described by using some related-art literatures regarding configurations that functionally operate in a way similar to that of a configuration of an LED driver circuit proposed by the inventor herein, that is, regarding adjustment of the brightness of an LED by using PWM control.
JP 2007-317443(A) has proposed a circuit that enables continuous dimming from the lower limit to the upper limit of an output without complicating circuitry in an illumination system for which dimming control is performed by controlling on/off of power from a power supply based on a PWM signal. In this illumination system, a PWM signal is supplied to a gate of an FET (field effect transistor) that controls an LED from a microprocessor including a ROM (read-only memory) to control the brightness of the LED. In the case where a pulse-on time of the PWM signal based on a control command is finer than a resolution of the clock of the microprocessor, a plurality of pulses having different on times are combined together to create a combined PWM signal so that the average of the pulse-on times becomes equal to the pulse-on time based on the control command. In this way, the continuous dimming from the lower limit to the upper limit of an output is realized. However, in JP 2007-317443(A), pulse width modulation for a low illuminance region and pulse width modulation for a high illuminance region are not used as control signals in order to increase a dynamic range for a LED brightness change, which is aimed by the present invention. Thus, JP 2007-317443(A) does not aim to smooth the brightness change in the low illuminance region.
JP 2011-171231(A) aims to provide an LED lighting circuit capable of performing dimming control in a range below the lower limit of PWM-based dimming. To this end, the LED lighting circuit includes a step-down chopper circuit that supplies an LED current to an LED light source unit as a result of oscillation control of a switching element, and a dimming control unit that controls the LED current by performing oscillation control of the switching element. The dimming control unit includes an oscillation frequency control unit that controls an oscillation frequency, a PWM control unit that controls a PWM on-duty, and a driver unit that switches on/off the switching element on the basis of the oscillation frequency and the PWM on-duty. Operation details are as follows. In a range in which the dimming degree is at or above the lower limit of PWM-based dimming, the PWM control unit changes the PWM on-duty to perform dimming control. In a range in which the dimming degree is below the lower limit of PWM-based dimming, the PWM on-duty is kept unchanged and the oscillation frequency is set to be higher than that for the lower limit of PWM-based dimming. The brightness of an LED is controlled by inputting a signal to the control terminal of the switching element from the driver unit to which the PWM signal from the PWM control unit and the oscillation frequency controlled by the oscillation frequency control unit are input. As in JP 2007-317443(A), pulse width modulation for a low illuminance region and pulse width modulation for a high illuminance region are not used as control signals, and control for mainly changing a pulse width that is input to a driver that causes flow of a small current in the low illuminance region and for mainly changing a pulse width input to a driver that causes flow of a large current in the other illuminance region is not performed.
An apparatus for generating a drive signal for an illumination device as described in JP 2013-519988(A) aims to provide a concept for driving an LED or an LED spot for an HDTV (high-definition television) camera while making requirements for a drive signal generator for the LED or the LED spot lower than in the related art. To this end, the apparatus according to JP 2013-519988(A) includes a pulse generator that generates a first pulse train in response to a first brightness request for a first brightness and generates a second pulse train in response to a second brightness request for a second brightness. The first pulse train has a first frequency, and the second pulse train has a second frequency different from the first frequency. The second pulse train includes two neighboring pulses of the first pulse train and an additional pulse between the two neighboring pulses. The additional pulse is not included in in the first pulse train. Operation details are as follows. Two pulse trains having different frequencies, the additional pulse being inserted to one of the two pulse trains, are input to drive the LED or the LED spot of the HDTV camera. However, this configuration is different from the configuration of the LED driver circuit proposed by the inventor of this application. In addition, the object is not to address the operation in which a brightness change is perceived as a stepped change in a very low light amount region or light abruptly goes out when the brightness of the LED is changed by using PWM control.